1. Field of the Invention
The present invention relates to a method for setting a machining feed rate of a machine tool, in which a workpiece is machined by a machining means based upon a machining program, and to the machine tool using that method.
2. Description of the Related Art
Conventionally, there have been two known methods for setting a machining parameter (e.g., a machining feed rate) so as to machine a workpiece to a certain contour shape in accordance with that machining parameter by a numerically controlled machine tool (i.e., an NC machine tool) on which a computer numerical controller (i.e., a CNC) is installed. According to one of the two known methods, a tentative machining feed rate is set based upon an NC data read in advance so as to prevent an acceleration in a radius direction of an arc interconnected with adjacent three points from exceeding a predetermined value. According to the other one of the two known methods, a tentative machining feed rate is set based upon an NC data read in advance so as to prevent a speed differential in a direction of each moving axis due to a bent angle at a central point among adjacent three points from exceeding a predetermined value. A machining feed rate can be set through some processes such as an accelerating/decelerating operation prior to an interpolating operation, a feedforward operation and the like after setting the tentative machining feed rate by use of the above-described method. That is, the machining feed rate is set through the optimum method in order to achieve a desired machining precision and a desired machining efficiency (i.e., a desired machining time), for example by use of a single method of the above-described methods or by combining the above-described methods.
According to the aforementioned conventional methods for setting the machining feed rate, the tentative machining feed rate is set based upon a local data among the adjacent three points on the workpiece (e.g., a contour shape) in stead of recognizing an entire workpiece or a part of the workpiece. Therefore, values of the machining speed rate may largely fluctuate even within a particular machined range such as a straight area or a curved area each of which contour shape variation is small. This problem occurs due to the following factor. A radius of an arc interconnected with the adjacent three points or the bent angle may fluctuate due to a rounding error raised when generating the NC data in a unit of 1 μm or 0.1 μm.
For instance, when a smooth arc shape, which is generated by approximating a straight line, or a free-form surface is machined, the tentative machining feed rate is set only based upon adjacent three local points according to the conventional method for setting the machining feed rate. In this case, values of the tentative machining feed rate may largely fluctuate. That may lead to a fluctuation of a final machining speed rate. Therefore, the workpiece to be machine may not be capable of being machined with an optimum machining precision. Further, the machining feed rate may be decelerated in comparison with a machining feed rate set based upon the whole workpiece to be machined.
In the conventional method, a differential value of the acceleration in the moving axis is changed on an arc shape portion with a small degree, because the machining feed rate is set based upon the adjacent three local points on the work portion. Therefore, an optimum machined surface can be obtained. However, a differential value of the acceleration in the moving axis is largely changed on a portion transforming from the arc shape to a straight shape, e.g., at a shape change point. In this case, an optimum machined surface may not be able to be obtained. To overcome this problem, the machining feed rate at the workpiece to be machined is required to be, as a whole, decelerated by reducing a setting value (i.e., a parameter) for setting the machining feed rate.
For instance, there is a case that a crossed edge surface with a fine angle (hereinafter referred to as a fine angle edge) is required to be machined accurately and sharply. Here, the fine angle is defined by two crossing flat surfaces, and is may especially set to be substantially equal to 3° or smaller than that. However, if the fine angle edge is machined by the above-described method for setting the machining feed rate, the parameter for setting the tentative machining feed rate is required to be extremely small so as to reduce the whole machining feed rate. As a result of the reduction in the whole machining feed rate, the machining efficiency may be hence deteriorated.
Accordingly, the present invention therefore seeks to provide an improved method for setting the machining feed rate of the machine tool which is capable of setting an optimum machining feed rate corresponding to the machined shape and to provide an improved machining apparatus provided with the improved method for setting the machining feed rate.